1. Field of Invention
The present invention is generally related to systems and methods for treating refractive errors in human eyes using wavefront-guided or customized laser ablation surgical techniques. In particular, the present invention is related to the use of preoperative manifest refraction information in combination with known preoperative wavefront information to improve the efficacy, or outcome, of using laser ablation techniques, such as LASIK, on human eyes, the efficacy/outcome being measured by postoperative results.
2. Description of the Related Art
One of ordinary skill in the art will understand that myopia refers to a refractive defect of the optical properties of an eye that causes images to focus forward of the retina (i.e., a refractive error). Those optical defects are typically caused by, among other things, defects of the cornea, elongation of the eye structure, other conditions, or a combination of those conditions. Hyperopia, on the other hand, refers a refractive error of the optical properties of an eye that causes images to focus behind the retina. Those optical defects are the result when the optics of the eye are not strong enough for the front to back length of the eye. Astigmatism (or “cylinder,” which are used interchangeably) refers to a refractive error that causes light entering the eye to focus on two points rather than one. It is caused by an uneven power of the cornea. Myopia, hyperopia, and astigmatism are the principle refractive errors that cause persons to seek treatment to correct their vision problems.
A manifest refraction analysis is a diagnostic tool used by ophthalmologists whereby a person's refractive error is determined as a means for indicating whether the person would benefit from correction with glasses or contact lenses. As part of that technique, a person looks through a phoropter while the ophthalmologist evaluates each of the person's eyes. A retinal reflex diagnosis technique is often used to assess the magnitude of the refractive error present in the person's eyes. Subjective feedback from the person is used to refine the manifest refraction, which involves the person making choices between image quality as different lenses having different powers are slid into place in the phoropter. At the end of the manifest refraction analysis, a prescription for glasses, contact lenses, and/or refractive surgery may be produced.
It is well known that a wavefront analysis of a person's eyes, using a wavefront sensor like the Zywave® Aberrometer made by Bausch & Lomb, Rochester, N.Y., and pioneered by Dr. David Williams and his team at the University of Rochester, can provide information about the person's visual acuity beyond that which the manifest refraction technique can provide. The wavefront analysis produces a shape of the wavefront of the person's eyes; the shape being described using Zernike polynomials. The polynomial shapes are classified as lower- or higher-order, based on the aberrations of the refracting optics of the eyes. Lower order aberrations, which most people with refractive errors have, consist of the 2nd-order aberration called defocus (i.e., myopia, hyperopia, and astigmatism). Those errors are correctable with glasses, contacts, or interocular lenses. Higher order aberrations, which some people have but in much smaller and varying amounts, consist of the 3rd-, 4th-, 5th-, . . . , nth-order aberrations. Higher order aberrations are aberrations of the optics of the eye above and beyond myopia, hyperopia, and astigmatism and they are not typically correctable with glasses or contact lenses.
Several refractive surgery techniques have been developed for correcting the higher order refractive errors in a person's eyes. U.S. Pat. No. 6,814,729 describes a refractive surgery technique using a laser. It teaches that a programmed series of ablating laser pulses are directed onto a patient's eye to reshape the cornea in an attempt to correct a refractive defect of the patient's eye. As noted above, the determination of a particular refractive defect starts with manifest refraction diagnostic information about the patient's eyes and its visual quality. That diagnostic information can be generated by one or more diagnostic devices including wavefront sensors, topography devices, ultrasonic pachymeters, optical coherence tomography (OCT) devices, refractometers, slit lamp ophthalmoscopes (SLOs), iris pattern recognition apparatus, and others that are well known in the art, and by other pertinent information that may be supplied by the surgeon, including surgical environmental conditions, particular patient data, surgeon-specific preferences, and others. According to the above patent, the appropriate input data are then fed to a calculation module in the laser system, which comprises software that uses the input data to determine an appropriate myopia, hyperopia, and astigmatism treatment.
One of the more common laser vision correction techniques is LASIK (i.e., laser-assisted in situ keratomileusis), which is a surgical procedure performed by ophthalmologists using ablation to remove corneal tissue and reshape the optics of the person's eyes. The LASIK laser is guided over the surface being exposed to the laser radiation in accordance with information that was inputted into the computer that operates the laser tracking system. That information is based on, as noted above, the aberrations identified in the person's eyes using a wavefront sensor, such as the Zywave® Aberrometer. Thus, the LASIK surgical procedure is often referred to as “wavefront-guided” laser ablation, and it is often marketed as being “customized” to the person receiving treatment. Customized LASIK has been shown to be effective in treating both the lower order (i.e., sphere and cylinder) and higher order aberrations (i.e., 3rd order and higher).
Other laser ablation surgical techniques are PRK (i.e., photorefractive keratectomy), EpiLASIK, and LASEK (i.e., laser epithelial keratomileusis); however, for purposes of this disclosure, the preferred embodiments of the invention will be described in context with the LASIK procedure.
A more complete technical summary of LASIK surgical procedures is contained in U.S. Patent Application Publication No. 20060017990 as follows: a) a Shack-Hartmann wavefront sensor is used to measure the aberrations in an optical system such as a living eye; b) a nomogram of the light-adjustable cornea's response to irradiation is then consulted to determine the required intensity profile to correct the measured aberrations; c) the required intensity profile is placed on a static mask (e.g. an apodizing filter) or a programmable mask generator (such as a digital mirror device); d) a calibration camera is used in a closed loop operation to correct the digital mirror device to compensate for aberrations in the projection optics and non-uniformity in the light source; e) the cornea is irradiated for the prescribed duration using the appropriate wavelength, intensity, and spatial profile; and f) after a specified diffusion time, the aberrations in the optical system are re-measured to ensure that the proper correction was made. If necessary, the process is repeated until the correction is within an acceptable pre-operative prediction target.
Several studies have reported the safety and efficacy of customized LASIK treatment for myopia. Despite being an advanced technology, it has been found that 24.1-percent of the eyes treated using customized LASIK have postoperative spherical equivalent (SE) of more than ±0.50 diopters (D), and about 10-percent of those eyes require re-treatment (a diopter is a unit of measurement for the power of a lens or of the refractive error measured in an eye). The etiology of postoperative refractive error has been associated with the corneal healing response and laser ablation characteristics.
One LASIK device, the Zyoptix® Custom Ablation system available from Bausch & Lomb, has been in use for several years after completing clinical trials in connection with a U.S. Food & Drug Administration (FDA) premarket approval application (the FDA approved the Zyoptix® system application on Oct. 10, 2003). One of the joint inventors of the present invention led one of the three FDA clinical trial centers involved in the study of the Zyoptix® system. The results of the clinical trials established the efficacy of the Zyoptix® system as follows.
First, 91.5-percent of the trial patients undergoing wavefront-guided laser eye surgery according to the Zyoptix® system had unaided vision of 20/20 or better (i.e., vision without glasses or contacts). Second, 70.3-percent of the trial patients had unaided vision of 20/16 or better. Third, more than 94-percent of the patients maintained or improved from their best-corrected vision with glasses six months post-operatively. Six months after surgery with the Zyoptix® system, 99.0-percent of subjects reported that they were moderately or highly satisfied with their results and 99.7-percent indicated improvement in quality of vision, of which more than 40-percent reported improvement in night vision while driving. None of the patients in the clinical trial reported dissatisfaction with their vision after surgery. Thus, the Zyoptix® system was shown to provide better postoperative refractive outcome than previous vision correction systems.
Despite those impressive results, it was found that the Zyoptix® system, as well as other laser ablation platforms, continued to cause overcorrection of the refractive errors when the system was operated strictly according to the predicted phoropter refraction (PPR) values set by the manufacturer in the programming of the system. Thus, in the case of the Zyoptix® system, the manufacturer determined that the laser output should be reduced by 93-percent to account for that overcorrection (the 93-percent was estimated from a linear regression of the discrepancy between postoperative results and the PPR values). Similarly, in the case of Alcon's LADARVision® system, the manufacture began recommending that the spherical correction be reduced to reduce incidences of overcorrection.
As noted above, the Zyoptix® system, like other systems, contains a set of preprogrammed instructions that may not be suitable for every person undergoing treatment. Surgeons are constantly developing personalized nomograms based upon relevant outcome-influencing factors that they have determined will optimize their treatment outcomes. For example, as described in U.S. Pat. No. 6,814,729, the Zywave® aberrometer, which includes a computer that runs software known in the industry as Zylink® ablation computation software, uses wavefront diagnostic data to determine an appropriate laser shot file for execution by a laser platform such as a Technolas 217Z® laser, also available from Bausch & Lomb. The patent describes a surgeon in Hong Kong that modified the software algorithm by incorporating a customized nomogram that produced optimized myopic correction for Asian patients, and a surgeon in Florida that obtained optimized surgical outcomes using a different myopia treatment nomogram that compensated for humidity effects on outcome. Thus, in addition to output adjustments recommended by the laser manufacturers to reduce incidences of overcorrection (or undercorrection), such as the 93-percent adjustment noted above for the Zyoptix® system, surgeons have been further adjusting their laser outputs and spherical correction calculations to account for various other site-specific factors.
The Kent-Mahon equation took this adjustment technique one step further by accounting for PPR wavefront refraction, sphere and cylinder refraction, but does not use manifest refraction information for sphere and cylinder to adjust the parameters of a laser vision correction systems in order to further refine the treatment and improve the outcome of patients undergoing refractive error surgery.
Similarly, in Bausch & Lomb's U.S. Patent Application Publication No. 20050251115, a method for making a diagnostic measurement to determine lower (second Zernike order or below) and/or higher (third and higher Zernike order) optical aberrations is disclosed in which an adjustment is made to a prospective photorefractive treatment based upon an expected, observed, calculated or otherwise anticipated biodynamical and/or biomechanical effect. Such an effect induces a deviation from an expected result of the prospective treatment in the absence of such biodynamical and/or biomechanical induced deviation. This adjustment, according to the patent application, will advantageously be a calculated or derived adjustment, however, empirical adjustments are entirely suitable as they form a basis for building and/or validating biodynamical and biomechanical models of the eye. That patent application, and other patent disclosures reviewed here to date, do not teach using the nomogram of the present invention.
In fact, to date, neither the Zyoptix® system nor previous known nomograms based on other laser platforms (e.g., VISX®, Alcon's LADARVision®, Zeiss' Meditec, Nidek, Wavelight Laser Technologies, Schwind, and LaserSight, among others), nor any previous known adjustments to laser system manufacturer's preprogrammed instructions, provide the surgical outcomes according to the present invention. Thus, it should be apparent that there exists a need for such a nomogram. In particular, it would be desirable to have a nomogram, based on both lower- and higher-order aberrations information and the interactions between higher and lower order aberrations, that is directed to treating refractive errors using wavefront-guided laser ablation techniques in which the refractive error correction efficacy consistently achieves 20/20 uncorrected vision in a higher percentage of patients than previous methods.